Radio telephone signaling



Dec. 26, 1933. E, H. ARMSTRONG 1,941,447

RADIO TELEPHONE SIGNALING Filed May 18, 1927 s sheets-sheet 1 AMPLIFIER FILTER Incoming Freqency.

Fig. l;

AMPl-I HER In ven tor Edwin H. Armstrong.

' Attorneys Dec. 26, 1933. E. H. ARMSTRONG 1,941,447

RADIO TELEPHONE SIGNALING Filed May 18, 1927 s Sheets-Sheet 2 FILTER AMPLIFIER FILTER 32 as 3) 31/ g A A l/3'9 B A} In ven tor L'dw/h H. Armsfrong.

Attorneys Dec. 26, 1933. E. H. ARMSTRONG 1,941,447

RADIO TELEPHONE SIGNALING Filed May 18, 1927 3 Sheets-Sheet 3 Fig. 5.

In ven tor Edwin H. Armsfronq Attorneys Patented Dec. 26, 1933 UNITED STATES PATENT OFFICE Claims.

This is an invention for eliminating the effects of fading and static in radio telephone transmission.

This is accomplished by a new method of 5 transmission in which the frequency of the transmitted wave (not its amplitude) is varied in accordance with the voice frequency to be transmitted and by a new method of reception in which only the frequency variations of the received wave arere-translated into voice frequencies, the effects of fading being thus prevented from appearing in the translated signals.

Fading manifests itself in a number of ways. One is a gradual change in the efficiency of the transmission path over a considerable period of time. The change in amplitude of the received signal in this case is so slow that this form does not interfere with the quality of the transmission. Another form manifests itself in rapid fluctuations of the efficiency of transmission between the sending station and the receiving station and this form does interfere with the quality since the fluctuations due to fading are so rapid as to give rise to currents of frequencies within the audible range.

As a solution of the first form of fading, it has been proposed to use high power at the transmitter to keep the low points of the transmission above the static level and to use automatic regulators of the amount of amplification at the receiver to compensate for the change in the strength of the received signal. This is effective within limits.

No satisfactory solution has been proposed for the second type of fading and it is to the solution of this particular problem. that this invention is directed, though the aplication of the invention simultaneously takes care of the first form also.

The effect of atmospheric disturbances is to introduce currents of variable amplitude and of a frequency which the tuning arrangements will admit. The extent of the disturbance depends among other things on the width of the band to which the receiver will respond.

The invention will be more readily understood by a reference to the drawings.

Figure 1 illustrates a preferred form of transmitting apparatus.

Figure 2 illustrates a preferred form of receiving apparatus.

Figure 3 shows a characteristic curve of the receiver.

Figure 4 illustrates some receiver details.

Figure 5 illustrates some transmitter details.

In Figure 1, reference characters 1, 2, 3, 4, 5, represent an oscillator system of usual type coupled to the modulator by coil 6. The grid of the modulator is controlled by the microphone system 9, 10, 11. Coupled to the oscillator by coil 12 is a high frequency amplifier 13 which feeds into a shielded grid type vacuum tube 16 by means of the transformer 14, 15. The shielded grid tube is adapted to operate as a current limiting device by controlling the emission of 66 its filament by the rheostat 17, as well understood. The output of the current limiter tube is supplied to a filter 20 which passes the desired band of frequencies but eliminates all harmonics resulting from the limiting action of the tube 16. The current which passes thru the filter 20 is supplied to a power amplifier 23 and theamplified currents arethen supplied to the antenna system 25, 26.

The operation of the transmitter is as follows. 76 The oscillator 1 to 5 determines the frequency of the band on which it is desired to transmit. The modulator system 6, 7, 8 is coupled sufficiently closely to coil 1 to modify the reactance of coil 1 a substantial amount, and therefore the adjustment of the oscillator to the mid frequency position is made by adjustment of the condenser 3. The voice frequency currents in the microphone circuit 9, 10, 11 then vary both the frequency as well as the amplitude of the oscillator current. By passing these oscillator currents thru the amplifier 13 and by supplying them to the tube 16, the amplitude variations are eliminated, leaving only the frequency variations plus some harmonics created by the limiter. By next passing the output of the tube 16 thru a band pass filter of the proper width the harmonics are eliminated and the currents which pass thru the filter may then be supplied to a power amplifier and an antenna in the usual way. The result is the transmission of a band of frequencies of constant amplitude; the frequency (at any instant) within the band depending upon the amplitude and the sense (positive or negative) of the swing of potential of the grid of the modulator tube.

Referring now to Figure 2 which illustrates the receiving arrangement, 30, 31 is the usual antenna arrangement, 32 a band pass filter, 33 an amplifier, 34 a current limiting device, 35 another filter, 36, 3'7, 38, 39, 40, 41 a selector circuit whose operation will be described later; 42 and 43 amplifier tubes coupled respectively to detectors 48, 49, arranged to operate the telephones 54. The resistances 50, 51 are adapted to cause the rectifiers to operate substantially in accordance with the straight line law and the resistances 52, 53 form a bridge arrangement with the telephones and operate in a manner which will be described later.

The operation of the system is as follows. The signal as it leaves the transmitting antenna is of varying frequency but constant amplitude. When it arrives at the receiver it has, at any instant, the same frequency as at the transmitter, but an amplitude which varies over wide limits. This signal is passed by the filter 32, amplified to a high value by the amplifier system 33 and supplied to the limiter tube 34 which removes the amplitude fluctuations introduced by fading and thus recreates the original signal. It then remains to convert the'frequency variations back into the original voice frequency currents, by providing a system which will respond selectively to frequency and yet act instantly without lag or resonance effect. In other words, it is necessary to parallel the action of "a rectifier system which converts variations of amplitude into voice frequency currents without resonance effects by a selective system which converts variations of frequency into voice frequency currents and yet which acts equally without resonance effects or lag.

Such a system is illustrated by the arrange- .ment 36 to 54.

Condensers 39 and 40 are so adjusted with respect to the inductance 41 that the circuit between AC is non-reactive for the highest frequency of the band which is transmitted and between 3-0 is non-reactive for the lowest frequency which is transmitted. Under these conditions if the effect of resistance in the condensers 39 and 40 and the coil 41 is disregarded, or, if, as illustrated in Figure 4, it is eliminated, the system has the characteristic shown in Figure 3. In this figure F1 and F2 represent the non-reactive points. The characteristic is an ideal one based on the assumption that the rectifiers follow the straight line law. The figure illustrates what may be approximated.

The action of the circuit arrangement 36 to 54 may be explained as follows: Any variations that may remain in the amplitude of the received current despite the action of the current limiting device 34 will manifest themselves as changes in the voltage drops between A and C, and B and C respectively. Under the conditions of adjustment of condensers 39 and 40 to the inductance 41 hereinabove specified, these voltage drops (and therefore the changes therein) are substantially equal in magnitude though different in phase. Being equal in magnitude, they balance each other in their effect on the telephone receiver, their difference in phase being rendered inconsequential because of the action of the rectifiers 48, 49.

Amplitude variations of the carrier are thus balanced out in their effect on the receiver.

Frequency variations in the carrier, however, are not balanced out by this circuit arrangement. The reason for this is that the combination of impedances between A-C responds differently in changes of voltage drop therein due to changes in frequency of the received current, from that between BC. In the specific instance illustrated, the drop A-C varies from a given value for one limiting frequency of the band to zero for the other, and across 3-0, from zero at said one limiting frequency to said given value at the other. These voltage drops, being thereupon caused to oppose each other in their effect on the receiver, set up a current therein which varies in accordance with the variation in frequency of the carrier.

The circuit arrangement of the present invention, therefore, is one which although fully responsive to the frequency variations is substantially ineffective to respond to amplitude variations of the received wave. Since static, fading and like disturbances manifest themselves substantially as amplitude variations of the wave, it will be apparent that such disturbances are greatly reduced in the present system.

It will thus be seen that one essential feature of the present system is'the provision of a circuit arrangement, diiferent portions of which respond differently to variations in frequencies of the current, but similarly to variations in amplitude of such currents. These responses are thereupon combined so as to oppose each other in their effect on a combined translating instrument, from which instrument the signal is derived.

Referring again to Figure 2, 36 and 38 may be adjusted to be non-reactive to the mid frequency F3 and 3'7 may thereafter be adjusted to make the complete circuit substantially aperiodic.

Referring now to Figure 4 there is illustrated one method of compensating the resistance of the condensers and coil of the selector circuit. 59, and 61 correspond to 39, 40 and 41 of Figure 2. 62 is a small resistance which should not be larger than the resistance to be compensated and should preferably be smaller. 63 is a vacuum tube' amplifier. 64 is a large resistance in the plate circuit of this tube. By properly adjusting the value of the plate resistance the resistance drop in the coil and condensers as manifested by the voltages between points A and C and between B and C disappears and the ideal condition is obtained.

In Figure 5 is illustrated a method of frequency modulation in which the effect of simultaneously modulating the amplitude is practically eliminated by a compensation method. In this arrangement to 77 represents an oscillator similar to that of Figure 1. Two modulator tubes 79 and 80 are employed, one connected across a condenser '73 in the tuned circuit, the other connected across an inductance '72. The grids of the two modulators are connected to the modulating transformer 81, 82 differentially so that when one goes positive the other goes negative. The reactances of the conductor 73 and inductance '72 are made small with respect to the other parts of the circuit. Also the reactances of 72 and 73 are adjusted to be equal to each other. Under these conditions the amplitude modulation by one tube is neutralized by the amplitude modulation of the other but the frequency modulation effects of both tubes are additive. The output of this oscillator is supplied to the amplifier and current limiting devices in the same manner as in Figure 1.

It will be observed that this method of modulation is not subject to the usual limitations which requires at least 5000 cycles. The band may be made any width desired depending on the particular conditions and the distance over which it is desired to operate. mined by experiment. In general however the narrower the band the less the effect of atmospheric disturbances. On the other hand, the band cannot be made too narrow when fading This can only be deterdifliculties are the main ones as the frequency fluctuations introduced by fading are then a factor. The proper choice is a compromise.

What I claim is:

1. In a device for receiving a frequency modulated wave, means for amplifying the received wave, means for so limiting the amplitudes of said wave that variations in amplitudes are eliminated, and frequency responsive means for translating the frequency variations of said wave into voice frequency currents, said means comprising a selective circuit, a portion of which is non-reactive for the highest frequency of the received band, and a second portion of which is non-reactive for the lowest frequency of said band, a plurality of detecting devices, means for impressing the potential difference across said one portion on one of said detecting devices and for impressing the potential difference across said other portion on the other of said detecting devices, and means for differentially combining the output currents of said detectors.

2. A radio receiving apparatus comprising means for picking up frequency modulated waves, means for eliminating amplitude variations of said waves so that the individual wave cycles are of like amplitude, and aperiodic means comprising inductance and capacity in series for trans lating the waves into audio frequency currents.

3. In a system for receiving a frequency modulated wave, a circuit having a plurality of portions, impedances in said portions for rendering changes in the voltage drops therein substantially equal in magnitude for changes in amplitude of the current therein and unequal for changes in frequency of said current, and means responsive to the voltage drops in said portions.

4. A system as set forth in claim 3, in which there is included in advance; of the circuit referred to, means for amplifying the received wave and means for so limiting the amplitudes of said wave that variations in amplitude are substantially eliminated.

5. A system as set forth in claim 3, in which provision is made of a plurality of detecting devices, means for impressing the potential difference across one of the circuit portions referred to upon one of the detecting devices, means for impressing the potential difference across the other of the circuit portions upon the other of the detecting devices, and means for differentially combining the output currents of said detecting devices.

6. A system as set forth in claim 3, in which one of the circuit portions referred to is nonreactive for one limiting frequency of a predetermined band of frequencies, and the other of the circuit portions is non-reactive for the other limiting frequency of the band, and in which means is provided for differentially combining the effects of the-voltage drops in said circuit portions.

'7. A system as set forth in claim 3. in which the impedances referred to are reactance elements, and in which provision is made of means for eliminating the effect of the resistance of said elements.

8. A system as set forth in claim 3, in which one of the circuit portions referred to is included within the other.

9. A system as set forth in claim 3, in which the circuit as a whole is aperiodic.

10. The method of receiving frequency modulated carrier waves which consists in passing said waves through paths responding in the same sense to changes in amplitude of the carrier but in opposite sense to changes in frequency thereof and combining the responses of said paths to produce the signal.

'11. In combination, a circuit for receiving frequency-modulated waves, means for limiting the amplitudes of such waves to a predetermined value, and a detecting system compr ising means deriving and additively combining current components from changes in frequency of such waves and deriving but differentially combining current components from changes in amplitude thereof.

12. In combination, a circuit for receiving frequency-modulated waves, means for limiting the amplitudes of such waves to a predetermined value, and balanced means substantially non-responsive to variations in amplitude of the wave for detecting the frequency variations thereof.

13. In combination, means for receiving a frequency-modulated wave, the frequency variations of which extend over a predetermined band of frequencies, means for limiting the amplitudes of such wave to a predetermined value, and a detecting system for deriving the signal variations of the frequency-modulated wave, said system having the characteristic of responding at any given instant with current in one direction or the other depending upon whether the frequency at such instant is greater or less than the mid-frequency of said band, the amplitude of the current varying as the difference between the instantaneous frequency and said mid-frequency.

14. In combination, means for receiving a frequency-modulated wave, the frequency variations of which extend over a predetermined band of frequencies, means for limiting the amplitudes of such wave to a predetermined value, and a detecting system for deriving the signal variations of the frequency-modulated wave, said system having the characteristic of responding at any given instant with current varying in one direction or the other from a given value at the mid-frequency of said band, depending upon whether said instantaneous frequency is greater or smaller than said mid-frequency, the amplitude of the current varying as the difference between the instantaneous frequency and said mid-frequency.

15. In combination, means for receiving a frequency-modulated wave, the frequency variations of which extend over a predetermined band of frequencies, means for limiting the amplitudes of such wave to a predetermined value, and a detecting system for deriving the signal variations of the frequency-modulated wave, said system having the characteristic of responding at any given instant with current varying in one direction or the other from a given value at mid-frequency, depending upon whether said instantaneous frequency is greater or smaller than said mid-frequency, the amplitude of the current varying as the difference between the instantaneous frequency and said mid-frequency, and the value of such current being zero at at least one. frequency of such band.

EDWIN H. ARMSTRONG.

DISCLAIMER 1,941,447.-Edwin H. Armstrong, New York, N. Y; RADIO TELEPHONE SIGNALING. Patent dated December 26, 1933. Disclaimer filed March 25, 1936, by the patentee.

Hereby enters this disclaimer of claim 10 as said claim appears in the original Letters Patent.

[Qfllh'lll Gazette April 21, 1.936.] 

